1. Field of the Invention
The present invention relates generally to a terminating point detecting method. More specifically, the invention relates to a polishing method of a metal film employing a chemical mechanical polishing (CMP) and a polishing device therefor.
2. Description of the Related Art
In a fabrication process of a semiconductor device, a process step of polishing a metal film formed on the a semiconductor substrate (wafer) is performed. In order to performing polishing optimally, it becomes necessary to accurately detect a terminating point of polishing for terminating polishing. One prior art system for detection of the terminating point of polishing, employing a rotary bed, is disclosed in Japanese Unexamined Patent Publication (Kokai) No. Heisei 6-120183 (1994). FIG. 5 is a diagrammatic illustration showing the construction of this first prior art system. In the disclosed technology, a polishing pad 24 having an opening 24a is provided on a surface of a rotary bed 23 which holds a wafer 21 to be polished on a carrier 22. By means of the carrier 22, the wafer 21 is pressed onto the surface of the polishing pad 24. In this condition, the carrier 22 and the bed 23 are rotatably driven while supplying a slurry as an abrasive from a source 25 to the surface of the polishing pad 24 for performing CMP polishing for the surface of the wafer. At this time, ions in the slurry in the openings 24a of the polishing pad 24 are put into a conductive state through wiring layers on the bed side and the wafer side. Therefore, upon supplying power from a power source 26, a current thereupon is measured by an ammeter 27. Since the detected current value is variable depending upon the remaining film thickness on the wafer surface, the terminating point of polishing can be detected by monitoring the detected current.
A second prior art system for detection of the terminating point of polishing is disclosed in Japanese Unexamined Patent Publication No. Heisei 6-216095 (1994). FIG. 6 shows a construction for implementing this second prior art system. A rotation speed of a carrier 32 to be driven to rotate upon polishing of a wafer 31 is measured by a revolution indicator 36 of a motor 35. The rotation speed of the carrier 32 is controlled by a control unit 37 so as to be constant throughout the polishing operation. When polishing is performed under the foregoing condition, as the flattening of the wafer surface progresses, the torque required to be exerted on the carrier 32 becomes smaller. This torque is measured by a torque meter 38 serving as a polishing resistance measuring means. A saturated condition of the measured torque is detected as the terminating point of polishing. In FIG. 6, the reference numeral 33 denotes a bed rotatingly driven by a motor 33a and carrying a polishing pad on the surface, the reference numeral 34 denotes a slurry supply source.
There also has been proposed a system for detecting the terminating point of polishing by optically detecting a film thickness of the wafer to be polished or a surface condition thereof. Such prior art has been disclosed in Japanese Unexamined Patent Publication No. Heisei 7-283178 (1995), for example. In this latter system, an energy beam, such as an infrared light is supplied from a front surface side of the wafer to be polished to the back surface side. By detecting variation of energy passing through the wafer, the film thickness and therefore the terminating end of polishing can be determined. In this latter system, when the infrared light passes through the wafer, energy absorption at a specific wavelength is caused in atom and bonded atom. Therefore, by monitoring the energy absorption amount, the terminating point of polishing can be detected.
Yet another prior art system is disclosed in Japanese Unexamined Patent Publication No. Heisei 8-17768 (1996) which proposes to move the wafer to be polished about an optical sensor at the intermediate timing in polishing process and to detect the terminating point by measuring the wafer or the film being polished by optical method.
In the first prior art, i.e. of Japanese Unexamined Patent Publication No. 6-120183, a current during continuous polishing of wafers is not constantly maintained within a given range in any wafers. Therefore, it requires setting at every occasion which makes polishing operation troublesome. This is true also for the case in detection of torque variation in accordance with Japanese Unexamined Patent Publication No. 6-216095. The reason is that, in the first prior art, a given amount and a given concentration of slurry cannot be supplied into the opening of the polishing pad and fluctuation of the current value can be caused due to difference of a pattern of the wafer surface and the polishing pad. In the second prior art, even when dressing for regeneration of the surface of the polishing pad is performed, it is inherent to cause variation (shifting) of torque since fatigue is constantly caused on the surface of the polishing pad.
On the other hand, in the foregoing third prior art in accordance with Japanese Unexamined Patent Publication No. 7-283178, the layer thickness is detected on the basis of composition with respect to a specific film. However, it is difficulty to detect chemical composition of the objective film to be polished with high precision. Accordingly, it has been difficult to detect the layer thickness at high precision. The reason is that it is difficult to detect chemical composition of only one layer in the multi-layered and highly integrated wafer surface structure. On the other hand, upon detection of the layer thickness of different material, re-setting becomes necessary. Furthermore, in the fourth prior art of Japanese Unexaamined Patent Publication No. 8-17768, measurement of the wafer has to be performed with interrupting polishing operation thus adding extra time for measurement and resulting in lower throughput. The reason is that the carrier holding the wafer has to be moved from the position above the polishing pad to a position above an optical sensor.